1. Field of the Invention
The present invention relates to pay television systems, and more particularly to picture scrambling devices and picture descrambling devices employing the base-band method for use in the pay television systems.
1. Description of the Prior Art
Conventionally, various types of picture scrambling have been employed in pay television systems in order to ensure their service or to prevent their programs from being unfairly enjoyed on the side of terminal equipment. The methods of picture scrambling can roughly be classified into two types, the RF signal method and the base-band signal method. In the RF-signal method type scrambling, the amplitude of the television signal is suppressed during both the line and field flyback periods by 6 dB or 10 dB, whereas in the base-band signal method type scrambling, synchronizing signals are shifted toward the video signal and also the polarity of the video signal is inverted, for example. In addition, decoding and timing signals for a picture scrambling signal are in general amplitude-modulated and multiplexed on an audio signal that is a frequency-modulated signal, for transmission.
However, such conventional methods of picture scrambling are insufficient for concealment, because the methods can not fully prevent TV programs from being unfairly listened to and watched, or taped until it is threatening the profits of broadcasters. As another problem, since for release of the picture scrambling a decoding signal or timing signal is amplitude-modulated so as to be multiplexed on an audio signal, the amplitude-modulation component may be a hindrance to the audio signal, resulting in a deteriorated sound quality.